5 Common SMC Molding Defects (And How to Fix Them)

Sheet Molding Compound (SMC) is a widely used composite material in industries ranging from automotive to electrical, offering excellent strength-to-weight ratios and corrosion resistance. However, achieving flawless SMC parts requires precise control over the molding process. Even small errors can lead to quality issues that compromise part performance and appearance. In this article, we’ll explore the 5 most common SMC molding defects and provide practical solutions to prevent or correct them. For further reference and sourcing high-quality SMC materials, visit General New Material SMC & BMC.

1. SMC Voids

Description:
SMC voids are air pockets or cavities trapped inside the molded part. They often appear as surface bubbles or internal cavities that weaken the component and reduce structural integrity.

Causes:

• Inadequate compaction of SMC sheets during molding.

• Excessive resin content leading to poor flow.

• High mold temperature causing premature resin curing before the SMC fully fills the mold.

Solutions:

• Optimize the mold pressure to ensure complete filling. Typically, 40–80 bar is used for standard SMC parts.

• Preheat the mold to the correct temperature, generally 140–160°C, to improve flow without causing early gelation.

• Reduce the resin content slightly if the SMC is too rich, ensuring a balance between reinforcement and flow.

Proper attention to these parameters can significantly reduce SMC voids and improve part consistency.

2. SMC Shrinkage

Description:
SMC shrinkage occurs when the molded part contracts unevenly during curing, leading to dimensional inaccuracies, warping, or internal stresses. This can compromise both fit and function.

Causes:

• Uneven heating of the mold or inconsistent curing times.

• Incorrect fiber-to-resin ratio in the SMC sheet.

• Large part thickness variations that promote differential shrinkage.

Solutions:

• Ensure uniform mold temperature using multiple heating zones or proper thermal management.

• Use consistent SMC sheet formulations with correct fiber content to minimize differential shrinkage.

• Design parts with uniform thickness where possible to reduce uneven contraction.

By addressing both material and process parameters, manufacturers can effectively control SMC shrinkage and maintain dimensional accuracy.

3. Surface Defects in SMC

Description:
Surface defects in SMC include rough surfaces, scratches, pinholes, or resin-rich spots. These imperfections affect aesthetics and may require costly post-processing.

Causes:

• Worn or dirty molds that impart scratches or contamination onto the surface.

• Inadequate resin flow during molding, leaving dry fiber areas.

• Excessive release agent that disrupts surface finish.

Solutions:

• Regularly maintain and polish molds to ensure smooth surfaces.

• Optimize molding cycle parameters—temperature, pressure, and time—to promote even resin flow.

• Use appropriate release agents in controlled amounts to avoid interference with the surface finish.

Consistent process monitoring and preventive mold maintenance are key to minimizing surface defects in SMC.

4. Fiber Misalignment

Description:
Fiber misalignment occurs when fibers within the SMC sheet are not oriented as intended, leading to weak points and anisotropic mechanical properties.

Causes:

• Poor handling or stacking of SMC sheets before molding.

• Excessive flow or turbulence of material during molding.

• Uneven compaction pressure across the mold.

Solutions:

• Handle and store SMC sheets carefully to preserve fiber orientation.

• Adjust mold flow paths and venting to reduce turbulence during compression.

• Use progressive compaction to ensure even pressure and maintain fiber alignment.

Correct fiber orientation is essential for maximizing the mechanical performance of SMC parts.

5. Resin Starvation

Description:
Resin starvation occurs when the fibers are not fully impregnated, leaving dry areas within the part. This leads to weak spots and poor surface finish.

Causes:

• Insufficient SMC material supplied to the mold.

• Low mold temperature or rapid curing that prevents resin from flowing adequately.

• Inconsistent SMC sheet thickness or improper stacking.

Solutions:

• Ensure adequate SMC material is placed in the mold, slightly more than the theoretical requirement.

• Control mold temperature and cycle time to allow complete resin flow before curing.

• Monitor sheet thickness and stack layers carefully for uniform distribution.

Preventing resin starvation ensures strong, durable parts with consistent surface quality.

Conclusion

Manufacturing high-quality SMC parts requires careful attention to material properties, mold design, and process parameters. Common issues like SMC voids, SMC shrinkage, and surface defects in SMC can be mitigated through proper compaction, temperature control, and material handling. By understanding the causes and implementing the recommended solutions, manufacturers can improve both the structural integrity and appearance of SMC components. For high-quality SMC materials and further guidance on advanced SMC solutions, visit General New Material SMC & BMC.

Proactive troubleshooting not only reduces defect rates but also enhances production efficiency, ensuring reliable, premium-grade SMC parts for your applications.

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